1. Field of the Invention
The present invention relates to a high-output photomultiplier having a microchannel plate. More particularly, the present invention relates to such a high-output photomultiplier tube having a plurality of sequentially arranged and cascaded electron multiplier microchannel plates. Still more particularly, the present invention relates to such a high output photomultiplier having a unitary multi-layer ceramic body assembly.
2. Related Technology
Microchannel plates have been used in various devices to intensify low-level images. For example, in night vision devices, a photoelectrically responsive photocathode element is used to receive photons from a low-level image. The photocathode produces a pattern of electrons (hereinafter referred to as, "photoelectrons") which corresponds with the pattern of photons from the low-level image. This pattern of electrons is introduced into a microchannel plate, which by secondary emission of electrons in a plurality of small (or micro) channels produces a shower of electrons in a pattern corresponding to the low-level image. That is, the microchannel plate emits the photoelectrons along with proportional secondary emission electrons to form an electron shower. This shower of electrons at an intensity much above that produced by the photocathode is directed onto a phosphorescent screen. The phosphors of the screen produce an image in visible light which replicates the low-level scene. Understandably, because of the microchannel plate, the representative image is pixalized, or is a mosaic of the low-level image.
More particularly, the microchannel plate itself conventionally includes a bundle of very small cylindrical tubes which have been fused together into a parallel orientation. These small cylindrical tubes have their length arranged along the thickness of the microchannel plate. That is, the thickness of the bundle is not very great in comparison to its size or lateral extent. Thus, a microchannel plate has the appearance of a thin plate with parallel opposite surfaces. Each tube forms a passageway or channel opening at its opposite ends on the opposite faces of the plate. Also, each tube is slightly angulated with respect to a perpendicular from the parallel opposite faces of the plate so that electrons approaching the plate perpendicularly can not simply pass through the many channels without interacting with the plate.
Internally the many channels of the microchannel plate are each coated with a material having a high propensity to emit secondary electrons when an electron falls on the surface of the material. Also, the opposite faces of the microchannel plate are provided with a conductive electrode coating so that a high voltage can be applied across the plate. A voltage is also applied between the photocathode and the microchannel plate to move the photoelectrons emitted by the photocathode to the microchannel plate. Consequently, electrons produced by the photocathode in response to photons from an image travel to the microchannel plate in an electron pattern corresponding to the low-level light image. These electrons enter the channels of the microchannel plate and strike the angulated walls which are coated with the secondary electron emissive material. Thus, the photoelectrons from the photocathode, plus the secondary emission electrons in numbers proportional to the number of photoelectrons, exit the channels of the microchannel plate to impinge on a phosphorescent screen. Because the microchannel plate is supplying a considerable number of electrons which become part of the electron shower on the phosphorescent screen, the plate is designed to support an electrical current between its opposite face electrodes. This electrical current between the opposite faces of a microchannel plate is known as a "strip current" and a portion of which replaces the secondary emission electrons supplied by the microchannel plate. Thus, the magnitude of strip current controls the magnitude of the maximum electron shower on the phosphor screen. This strip current is also the source of the electrical resistance heating experienced by a microchannel plate.
Alternatively, rather than directing the electron shower from a microchannel plate to a phosphorescent screen to produce a visible image, this shower of electrons may be directed upon an anode in order to produce an electrical signal indicative of the light or other radiation flux incident on the photocathode. As will be further explained, a device making such use of a microchannel plate is generally referred to as a photomultiplier tube, although internally of the device, electrons are cascaded or multiplied rather than photons.
Still alternatively, such a microchannel plate can be used as a "gain block" in a device having a flow of electrons. That is, the microchannel plate provides a spatial output pattern of electrons replicating an input pattern and at a higher electron density. Such a device is useful, for example, to detect high energy particle interactions which produce electrons. Alternatively, such a device is useful as a particle counter when provided with an input element which sheds an electron when a particle of interest collides with the input element. The shed electron then stimulates the emission of secondary electrons, and an output current signal proportional to the number of particles is produced.
Conventional photomultiplier tubes are also known which make use of cascaded microchannel plates. That is, multiple microchannel plates are arranged in series so that the initial electrons from a photocathode, for example, fall into the first microchannel plate. From this first plate, the initial electrons and the secondary electrons from the first plate fall into a second microchannel plate. This second microchannel plate adds its own secondary emission electrons, and provides an increasingly intense shower of electrons. This shower of electrons may flow to a third or subsequent microchannel plate for further multiplication. In this way a very high electron gain or amplification may be effected, with each initial electron falling into the first plate resulting in several hundred to several millions of electrons flowing from the last microchannel plate of the cascade and to an anode. At the anode, the electron charge pulses are processed to count initial electrons, or to generate an image electronically, for example.
With the conventional photomultiplier tubes using cascaded microchannel plates, the electrostatic voltage is connected across the top electrode of the top microchannel plate and the bottom electrode of the last or bottom microchannel plate in the cascade. The microchannel plates of such a conventional photomultiplier tube are electrically connected in series. Thus, each of the microchannel plates in the cascade experiences the same strip current. The conventional photomultiplier tubes generally use resistance matched microchannel plates in order to control the voltage drop across each of the microchannel plates within the cascade. In other words, the last microchannel plate in such a cascade must have similar strip current as the earlier plates in order to provide the same level of electron multiplication. Conventional microchannel plate photomultiplier tubes are in part limited by the maximum output which can be sustained by the strip current of the last microchannel plate. The conventional approach would use cascaded high strip current microchannel plates to achieve high output, however, cascaded high strip current microchannel plates lead to excessive heating and thermal destruction of the cacaded plates and photomultiplier tube. In order to prevent such a thermal destruction of conventional photomultipliers, the cascaded microchannel plates are selected so that the cascade carries only a strip current which it can thermally sustain. However, this expedient understandably limits the performance of the conventional photomultiplier tubes in terms of their electron multiplication level.
A conventional microchannel plate is known in accord with U.S. Pat. No. 4,737,013, issued 12 Apr. 1988, to Richard E. Wilcox. This particular microchannel plate has an improved ratio of total end open area of the microchannels to the area of the plate. As a result, the photoelectrons are not as likely to miss one of the microchannels and impact on the surface of the microchannel plate to be bounced into another one of the microchannels. Such bounced photoelectrons, which then produce a number of secondary electrons from a part of the microchannel plate not aligned with the proper location of the photoelectron, provide noise or visual distortion in the image produced by the image intensifier. The image intensifier taught by the Wilcox patent solves this problem of the conventional technology.
Other specific uses of microchannel plates are in the image intensifier tubes as found in the night vision devices commonly used by police departments and by the military for night time surveillance, and for weapon aiming. However, as mentioned above, microchannel plates may also be used to produce an electric signal indicative of the light flux or intensity falling on a photocathode. In other words, if a single anode is disposed at the location ordinarily occupied by the phosphorescent screen, this anode will provide a current indicative of the photons received from a low-level scene. If the single anode is replaced with a grid or array of anodes, the various anodes will provide individual signals which are an electrical analogue of the image mosaic. Consequently, these electrical signals could be used to drive a video display, for example, or be fed to a computer for processing of the information present in the electrical analogue of the image.
In view of the above, it is easily understood that an image intensifier could be used as a detector for electronically detecting the occurrence of events which produce photons, such as collisions in a test chamber of a particle accelerator. When such an image intensifier is provided with an array of anodes, the occurrence of a signal at one of the anodes indicates the occurrence of an event, and the location and intensity of the signal can provide information about the event. An array of such detectors may be used to provide multiple indications of such events, and to provide comprehensive information about the events occurring in a large test chamber.